344 Mr* C. Barns on the Change of the Order of Absolute 



such configurations practically vanish as to number. I doubt 

 whether in soft steel the viscous deformation due to instanta- 

 neous exceptionally intense molecular motion at different 

 points within the solid, could be recognized at ordinary tem- 

 peratures. The case is different when the solid is under stress. 

 Any twist, no matter how small (?), is accompanied by a pro- 

 portionate amount of permanent strain, with which a cor- 

 responding amount of instability is necessarily associated. 

 Hence it must be borne in mind that the viscosities expressed 

 absolutely below refer to steel in a condition of strain just 

 within the elastic limits. 



I can here merely allude to the very recent and suggestive 

 paper of C. H. Carus-Wilson (Nature, xli. p. 213, 1890), in 

 which the behaviour of steel near the elastic limits is inter- 

 preted in analogy with the well-known circumflexed iso- 

 thermals of condensible gases, due to James Thomson. 



Now it is clear that if stressed steel be left to itself, the 

 number of unstable configurations becomes rapidly less as the 

 time after twisting increases. Energy is dissipated. The 

 viscosity of solids is therefore essentially a time-function as 

 well as a strain-function. 



12. The apparatus with which the following absolute results 



Fig-. 2. —Apparatus for measuring the Absolute Viscosity of Solids. 



d 



were obtained is shown in fig. 2. A A and BB are two 

 massive torsion-circles, the hollow cylindrical shafts, o o, of 



